Nasa astronauts have completed the last spacewalk of the space shuttle era.

The six-and-a-half hour excursion retrieved an ailing ammonia pump on the International Space Station (ISS), where shuttle Atlantis is docked.

The spacewalk is part of a busy schedule for the last shuttle mission, which will also see four tonnes of supplies delivered to the ISS.

More than a tonne is enough food to sustain the ISS crew for a year; the balance is spare parts and equipment.

Smaller supply ships developed by private companies are expected to supply the ISS when the shuttle programme retires.

The bumper crop of supplies delivered by Atlantis comes in anticipation of the end of the shuttle era, as it offer cargo capacity that currently no other spacecraft can provide.

The 160th spacewalk at the ISS has seen American astronauts Ron Garan and Mike Fossum - who were aboard the ISS before Atlantis arrived on Sunday - remove an ammonia pump from the station and place it in Atlantis' cargo bay.

The pump failed last year, disabling half of the station's cooling system.

The pair then attached the Robotic Refueling Mission - a joint US and Canadian experiment to test the idea of remotely repairing and refueling satellites using robots controlled from Earth.